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All EOR projects begin with an analysis of the nature, location, and causes of residual oil saturations (''S''<sub>or</sub>) that remain after primary and/or secondary recovery operations. The main factors that control the value of ''S''<sub>or</sub> are pore geometry, rock [[wettability]], and the properties of the displaced (oil) and displacing (injected) fluids. Fluid properties of particular interest are interfacial tension, viscosity, and density. In combination with the heterogeneity of the reservoir, these properties result in the overall recovery (''E''<sub>R</sub>) for any recovery scheme.
All EOR projects begin with an analysis of the nature, location, and causes of residual oil saturations (''S''<sub>or</sub>) that remain after primary and/or secondary recovery operations. The main factors that control the value of ''S''<sub>or</sub> are pore geometry, rock [[wettability]], and the properties of the displaced (oil) and displacing (injected) fluids. Fluid properties of particular interest are interfacial tension, viscosity, and density. In combination with the heterogeneity of the reservoir, these properties result in the overall recovery (''E''<sub>R</sub>) for any recovery scheme.
The overall recovery is the product of displacement efficiency (''E''<sub>D</sub>), and sweep efficiency (''E''<sub>S</sub>). The displacement efficiency is inversely proportional to the residual oil saturation, while the sweep efficiency is inversely proportional to the mobility ratio (''M'') between the injected fluids and the oil in place (see “[[Waterflooding]]”). ''M'' is usually stated in terms of the [[relative [[permeability]]]] of a fluid phase (''k''<sub>r</sub>) divided by the phase's viscosity (''μ'') relative to the same ratio for the other phase, such as for a waterflood:
The overall recovery is the product of displacement efficiency (''E''<sub>D</sub>), and sweep efficiency (''E''<sub>S</sub>). The displacement efficiency is inversely proportional to the residual oil saturation, while the sweep efficiency is inversely proportional to the mobility ratio (''M'') between the injected fluids and the oil in place (see “[[Waterflooding]]”). ''M'' is usually stated in terms of the [[Relative permeability]] of a fluid phase (''k''<sub>r</sub>) divided by the phase's viscosity (''μ'') relative to the same ratio for the other phase, such as for a waterflood:
:<math>M = (k_{\rm rw}/\mu_{\rm w})/(k_{\rm ro}/\mu_{\rm o})</math>
:<math>M = (k_{\rm rw}/\mu_{\rm w})/(k_{\rm ro}/\mu_{\rm o})</math>